What is the recommended space between displays in a video wall? Does it vary by manufacturer?

It should never be less than 0.050” or 1.27 mm. Typically the thickness of a standard business card is a good gauge!

What is the spacing tolerance built into the design of our model specific spacers?

We incorporate 0.050” or 1.27 mm into the spacer, but that is all contingent on the spacer being perfectly placed.
Horizontally there is much tolerance because the display slides side to side on the mount, but vertically it is more critical for proper placement.

What is the key for a successful installation of an LMV video wall?

1) Make sure all the mounting bracket settings are in 0 or neutral position.
2) Start the installation on the bottom row, working your way up.
3) Watch quick installation video on the LMV product page.